fifth wheel

Noun

fifth wheel (plural fifth wheels)

1. (US) A type of trailer hitch, which consists of a horseshoe-shaped plate on a multidirectional pivot, with a locking pin to couple with the kingpin of a truck trailer.
   • In most forms of semi-trailers a great deal of time is wasted in connecting the trailer to the tractor or pulling vehicle. Most of the connecting gears are some form of fifth wheel, consisting of two horizontal circular bands of iron, one fitting on top of the other, with grease between and with a...1919 August, “Connecting the Trailer in Record Time”, in Waldemar Kaempffert, editor, The Popular Science Monthly, volume 95, number 2, New York, N.Y.: Modern Publishing Company, […], →OCLC, page 28...
   • The semi-trailers are equipped with hydraulic brakes which operate through the king pin of the fifth wheel. The booster-controlled fifth wheel gives full and complete cab control of all braking and coupling operations.1934 August 25, “M.D.T. [Merchants Despatch Transportation Company] Develops Trailer Tank for Milk Service”, in Samuel O. Dunn, editor, Railway Age, volume 97, number 8, Philadelphia, Pa....
   1. (US) In full, fifth-wheel trailer: a large caravan or travel trailer that is connected to a pickup truck for towing by a hitch similar to the one described in sense 1 located in the center of the truck's bed.
      • Fifth-wheel hitching is a simple affair compared to trailer coupling. Choose a quality hitch and don't overburden an inadequate pickup truck with a big fifth wheel and you're almost in business.1997, Kim Baker, Sunny Baker, “Towing, Tow Vehicles, and Towed Vehicles”, in The RVer’s Bible: Everything You Need to Know about Choosing, Using, and Enjoying Your RV (A Fireside Book), New York...
      • Janet [Rosenstock] and Dennis [Adair] are avid travelers who venture to Europe now and again, and as full-time RVers travel through the United States and Canada. Look for them on the open road; their fifth wheel bears the sign "Read a Joyce Carlow lately?"2000, Janet Rosenstock, Dennis Adair, “About the Author”, in The Fire, the Sword and the Devil, Lincoln, Neb.: toExcel, iUniverse, →ISBN, page 382
2. (US, historical) A horizontal wheel or segment of a wheel above the front axle and beneath the body of a carriage, forming an extended support to prevent it from overturning.
   • Full fifth-wheels have become quite frequent of late, and it may not be out of place to make a few remarks on their merits as compared with half fifth-wheels. With the exception of heavy coaches and rockaways, nearly all vehicles were until lately made with half fifth-wheels; but at present we find...1873 April 1, H. F. Porter, “The Relative Merits of Full and Half Fifth-wheels”, in The Hub, volume XV, number 1, New York, N.Y.: Hub Publishing, →OCLC, page 10, column 1
   • For greasing the axles and fifth wheel, use castor-oil. It is not necessary to put on a great deal. [...] The fifth wheel should be looked after, and not be allowed to become entirely dry.1875, “The Proper Care of Carriages”, in The Coach-makers’ Illustrated Hand-book, […], 2nd edition, Philadelphia, Pa.: I[saac] D[elaney] Ware, […], →OCLC, part V (Miscellaneous), page 350
3. (US, idiomatic, informal) Anything superfluous or unnecessary.
   • I felt like a fifth wheel when both of them started giggling and making out during dinner.
   • A subjunctive mode should no more exist in the English grammar, than a fifth wheel be given to a wagon.[1818 December, “A Grammar of the English Language, in a Series of Letters. […] By W[illia]m Cobbett. New York, 1818. 1 vol. 12mo. pp. 184. 75 cts. [book review]”, in Oliver Oldschool [pseudonym...

   Synonyms:

   spare tool, third wheel

   1. (US, idiomatic, informal) An unwanted person accompanying two couples on a double date.